Looks like kdesudo double encodes the output. This can be fixed with

#! /usr/bin/env python3
import sys

for line in sys.stdin:
    sys.stdout.write(line.encode('raw_unicode_escape').decode("utf-8"))


Reproducible: Always

Steps to Reproduce:
$ sudo sh -c "echo привет"                                                 
привет

$ kdesudo sh -c "echo привет"
пÑивеÑ

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 14.10
Release:        14.10
Codename:       utopic


Kubuntu 15.10: exactly the same